NullPointerException continues

问题

When I am trying to call JPanel panel2 of Panel2 class on triggering of action event from Next JButton of Panel1 class, I am getting NullPointerException. How to resolve this? plzz help.

public class PanelEventTest
{

/**
 * @param args
 */
JFrame frame;

void originalFrame()
{
    frame = new JFrame();
    frame.setSize(500, 300);
    frame.setVisible(true);
    frame.setLayout(new FlowLayout());
    frame.add(new TestPanel1().panel1());
    frame.add(new TestPanel2().panel2());
}

public static void main(String[] args) {
    // TODO Auto-generated method stub
    new PanelEventTest().originalFrame();
}
}

public class TestPanel1
{
JPanel panel1;
JButton next;

JPanel panel1()
{
    panel1 = new JPanel();
    next = new JButton("Next");
    next.addActionListener(new ActionListener()
    {
        public void actionPerformed(ActionEvent e) 
        {
            // TODO Auto-generated method stub
            new TestPanel2().panel2.removeAll();
        }
    });
    panel1.add(next);
    return panel1;
}
}
public class TestPanel2
{
JPanel panel2;
JList jlist;
String[] list = {"Sachin","Tarun","Vipin"};
JPanel panel2()
{
    panel2 = new JPanel();
    jlist = new JList(list);
    panel2.add(jlist);
    panel2.add(new JLabel("Test"));
    return panel2;
}
}

My last question Nullpointerexception with JPanel was successfully resolved by you guys. Plz help in this. This exception is eating my head.

回答1:

If you try changing this line:

new TestPanel2().panel2.removeAll();

To:

new TestPanel2().panel2().removeAll();

Will solve the problem, but the current logic is flawed.

A better solution is:

Change TestPanel2 to:

public class TestPanel2 {
    JPanel panel2;
    JList jlist;
    String[] list = { "Sachin", "Tarun", "Vipin" };

    public TestPanel2() { // was: JPanel panel2() {
        panel2 = new JPanel();
        jlist = new JList(list);
        panel2.add(jlist);
        panel2.add(new JLabel("Test"));
                      // was: return panel2;
    }
}

And then modify TestPanel1 to:

public class TestPanel1 {
    JPanel panel1;
    JButton next;

    public TestPanel1(final JFrame frame, TestPanel2 tp2) { // was: JPanel panel1() {
        panel1 = new JPanel();
        next = new JButton("Next");
        final JPanel panel2 = tp2.panel2; // line created
        next.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                // TODO Auto-generated method stub
                panel2.removeAll(); // was: new TestPanel2().panel2.removeAll();
                frame.validate(); // line created
                frame.paint(); // line created
            }
        });
        panel1.add(next);
                         // was: return panel1;
    }
}

Finally, on PanelEventTest.originalFrame(), change:

frame.add(new TestPanel1().panel1());
frame.add(new TestPanel2().panel2());

to:

TestPanel2 tp2 = new TestPanel2();
frame.add(new TestPanel1(frame, tp2).panel1);
frame.add(tp2.panel2);
frame.validate();
frame.repaint();

Explanation

You are creating methods, when you needed constructors. You must read this: Understanding constructors.

Also, you need to pass TestPanel2 and the frame to TestPanel1:

Your code was creating a new TestPanel2 (attached to no one) and then calling removeAll() on it's pannel. This has no effect at all (as this panel is not shown anywhere).
The changed then code will call removeAll() on TestPanel1's panel.
Also, you need to revalidate/repaint the components everytime you make a change on them.
- Currently you change them when you create the frame (adding the panels) and when you remove the panel2 (in the "Next" button's action).

来源：https://stackoverflow.com/questions/15986208/nullpointerexception-continues

标签

java

swing

nullpointerexception

jpanel